Gas turbine engines include a combustion chamber wherein fuel is burned to supply energy that is then extracted by the turbine as mechanical work. To enable combustion, the fuel and compressed air are injected into a combustion zone within the chamber in such a manner as to cause mixing of the air and fuel. Usually the fuel is supplied through one or more fuel nozzles positioned at one end of the combustion chamber. The air is typically supplied by a plurality of air jets proximal the fuel nozzles and distributed along the body of the combustion chamber.
Ideally, the average temperature of the gases exiting the combustion chamber into the turbine is as close to the temperature limit of the material comprising the turbine components as possible. High temperatures are necessary in order to obtain maximum thermal efficiency. Because the fuel enters the combustion chamber and is burned at discrete locations within the combustion chambers, and because of various other practical limitations, it is not possible to achieve an exhaust gas temperature that is completely uniform. Instead, high local temperatures or hot spots in the gas stream will occur. Because the maximum temperature of the gas that reaches the turbine inlet must be below the temperature limit of the turbine components, the average temperature of the gas must be reduced to ensure that the maximum anticipated hot spot will not exceed the turbine temperature limit. Accordingly, the presence of these gas stream temperature anomalies results in a decrease in total gas energy and a corresponding decrease in engine efficiency.
Additionally, it is known that if the fuel-air mixture is not uniformly distributed throughout the chamber, unacceptable levels of CO, NOx and other unwanted gases are formed. In order to reduce objectionable gaseous emissions and improve temperature uniformity, it has been suggested to provide an air swirling device coaxial with each of the fuel nozzles. These swirlers cause the air to flow in a helical (rather than purely axial) direction about the fuel nozzle. Traditional swirler configurations, such as that disclosed in U.S. Pat. No. 5,373,693, establish what may be referred to as a circular flowfield at the swirler exit (as used herein, the term "circular flowfield" refers to a helical flowfields of circular cross-section). In multi-nozzle burners, such as an annular burner, the extent to which a circular flowfield provides optimal flow is inherently geometrically limited, because the circular flowfield provides limited nozzle-to-nozzle mixing. Closer spacing of nozzles improves nozzle-to-nozzle mixing, but only at substantial additional cost.
Accordingly, a need exists for an improved swirler for use in an annular combustor that maximizes nozzle-to-nozzle mixture flow within the combustor while minimizing the number of swirlers and fuel injectors needed for required combustor performance.